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For Most Of Human History, Being An Omnivore Was No Dilemma

Gorillas are fine with being herbivores, like this one at a Seattle zoo. But humans evolved as omnivores. Is diet destiny?
Ted S. Warren

If diet is destiny, then modern humans should thank our ancestors for their ability to eat just about anything.

Two new studies peek into the distant past to try to figure out just how big a role food played in human evolution. One says that eating meat made it possible for early human mothers to wean babies earlier and have more children.

The other study finds that humans and some other primates have stuck with being omnivores for a very long time. That's unlike many of our mammal friends, who used the omnivore lifestyle as a mere rest stop on the way from herbivore to carnivore.

"Primates are a little bit weird," says Samantha Hopkins, an assistant professor of geology at the University of Oregon, who led the study that revealed primates' omnivorous ways. Most primates became omnivores early in their existence, and stayed put. "We seem to hang out in this omnivorous role."

It's easy to imagine that there's an evolutionary advantage to being able to eat just about anything. Herbivores and carnivores have specialized teeth and digestive systems that make going back and forth practically impossible.

For instance, carnivores are usually the first to go extinct when times are tough, because they depend on other animals for their food source.

But there may be some evolutionary downside to being an omnivore, too Hopkins says. Namely, we're slow to diversify.

It took three times longer for omnivores to diversify, compared to herbivores. Producing more varied species means producing more progeny, which is the name of the game in evolution.

Hopkins and her colleagues found this out by scanning the literature for data on what 1,500 species of modern mammals eat. They gleaned it from field research by biologists, who sift through poop and examine stomach contents. It is not glamorous work.

They then matched the animals' diets with the mammalian family tree, and traced back the branches. It's the first study to look at diet across all mammal group through evolutionary time. The studywas published in the Proceedings of the National Academy of Sciences.

The second study looked at how long modern mammals nurse their young. Researchers in Sweden compared the diet, brain size, and weaning times of 67 species. Humans breastfeed for 2 years on average, while chimpanzees, our closest relatives, nurse for four to five years.

They found that all the animals stopped nursing when their brains hit a certain stage of development, regardless of diet. All the meat-eaters, including ferrets, killer whales, and humans, reached that point of brain development earlier than herbivores or omnivores, the researchers found. (They classified humans as carnivores based on the percentage of meat in the typical human diet.)

Also, they conclude, the big difference in breast-feeding times between humans and other primates is due to the better nutrition provided to both mothers and babies by meat consumption. The study was published online in PlosOne.

Big caveat: Both of these studies looked at the role of diet in evolution. They aren't a commentary on whether modern-day eating habits, carnivorous or not, are healthy.

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